The thickness of paper sheet is referred to as caliper and the caliper profile refers to the thickness profile across the width of the sheet. Caliper profiles are measured by scanning devices located downstream of a series of rolls arranged in parallel, one above the other in a stack. The sheet material passes through the space between adjacent rolls known as a nip. Such rolls are generally defined as calender rolls and the caliper profile is changed by adjusting the spacing between adjacent rolls, and by controlling the nip pressure and the surface roll temperature. These two control systems are interdependent and both systems are able to control in a series of zones along the length of the rolls to ensure that the paper sheets have a substantially uniform caliper profile across the paper width.
One example of a calender control system for sheet making is shown in U.S. Pat. No. 4,982,334, assigned to Measurex Corporation. As can be seen, the scanning device produces a signal representing caliper profile across the width of a paper sheet. This signal is then split into a high frequency signal and low frequency signal. The low frequency signal, which represents coarse control, is fed to what is referred to as a crown roll. This is a hollow roll containing internal hydraulic mechanisms to exert hydraulic pressure to change the curvature laterally along the length of the roll and thus the nip pressure is varied along the roll. In practice the hydraulic mechanisms can either cause the lateral surface of the crown roll to have a convex, flat or concave shape. In some cases sectionalized crown rolls are used, dividing the roll into a series of zones. However, in most cases the control system is reasonably coarse simply because it changes the shape of the roll overall.
The high frequency control is fed to a series of individual heaters positioned on zones extending along the length of a roll. The heaters apply heat to vary the surface temperature of the roll in each zone, and the roll expands or contracts in that zone to vary the paper sheet thickness in that zone. These heating devices may be used on a crown roll or on a plain steel roll, depending on the particular arrangement of rolls in a calender stack. There are generally two types of heaters used for heating the surface of the roll. One type is an induction heater, an example of such a type is sold by Measurex Corporation under the trade mark CALCOIL. A second type of heater is what is referred to as a heated air shower, and these heated air showers blow hot air in each of the zones along the length of the roll. The temperature of the air for each shower is controlled to vary the surface temperature of the roll and consequently vary the paper sheet thickness. One example of such a type of heated air shower is sold by Measurex Corporation under the trade mark CALTROL.
The heaters provide the fine adjustment, what is referred to as high frequency adjustment, and the internal hydraulic mechanisms in a crown roll provide the coarse adjustment, what is referred to as the low frequency adjustment. When the zone of a roll is heated, the diameter increases and thus the load on a paper sheet in a nip increases. This causes a localized decrease in thickness which maintains the thickness profile at a desired target.
After a paper sheet breaks, the temperature of the roll faces change significantly due to the fact that no paper sheet is passing therethrough. Thus, when the paper is rethreaded the thickness profile is often far off the predetermined thickness limits required for the paper, so the paper is not commercially saleable. There is therefore a requirement to return caliper profile for a paper sheet to within the predetermined limits in as short a time as possible, thus avoiding paper wastage.
Control systems presently used to control the caliper profile following signals received from a scanning device do not compensate for the process behaviour during heating. This results in control signal windup which can cause the process to overshoot the target. The initial error from target at start up is usually quite large and generally made up of large humps and valleys in the caliper profile. The low frequency thickness variation is ideally suited to the hydraulic load cylinders inside a crown roll which react quickly. However, the high frequency control which involves varying the heat of the roll surface has a slower reaction time. Once the low frequency target has been met, then the high frequency errors from the target are still too large and must be attacked with the finer control utilizing heaters. The high frequency control system is generally limited by the speed at which the roll surface can be heated. When heat is applied to one zone, the temperature and therefore the roll diameter, increases following a ramp trajectory. The shape of this ramp changes with the amount of heat applied, and the resulting effect is to have an overshoot, thus the thickness of the paper sheet in that zone becomes too thin. To avoid an overshoot, it is necessary to provide a system wherein the maximum reduction in the error of the caliper profile in one zone occurs, but the caliper in that zone does not go either under or over the target, namely the predetermined limits.
It is an aim of the present invention to provide a method of reducing start up times after a break in a paper sheet on a paper machine to bring the caliper profile across the width of the paper sheet to within predetermined limits as fast as possible. It is also an aim of the present invention to provide a control system which reduces start up time to bring the caliper profile to within acceptable quality limits after a machine break, and to co-ordinate the use of various actuators to control heaters and heating and cooling rates of rolls to reduce the recovery time or start up time.
It is also an aim of the present invention to use an adaptive control scheme to bring the caliper profile of a paper sheet to within the predetermined limits as fast as possible without overshooting the predetermined limits and thereafter to maintain control of the caliper within the predetermined limits.